Distributed mechanical systems such as micromachincd resonant strain gages poesess M in6nite number d modes of vibration. Mostly, one is interested in only one or a few modes. A method Is described with which only the desired modes are excited and detected. This is achieved by geometrically shaping the elements used for excitation and detection of the vibration. The method is based on the orthogonality principle, which k valid for a variety of structures and vibrations. In thii paper we have restricted ourselves mainly to transversal vibrations of prismatic beams, clamped on both sides (microbridges). The design rules for obtaining the shapes for most commonly used excitation and detection mechanisms are deduced and the e6eet of axial stress on tbe suppression of unwanted modes is discussed. The theory was veriled by experiments on resonators with selective mode excitation.using different excitation mechanisms, a i well as on resonators with both selective excitation and detection.

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The dependence of the relative frequency shift and the Q-factor of the first two modes of vibration of square silicon diaphragms on the diaphragm geometry and the air pressure is investigated. The experimental results are compared with the theory, which is based on Lamb's theory for circular diaphragms. It assumes acoustic radiation to be the determining mechanism for the energy loss of the diaphragm. The experimental results for both the frequency shift and the Q-factor as a function of the pressure deviate from the theory. A possible explanation for this deviation is the assumption that viscous damping plays an important role. It is found that the vibrating diaphragm is able to put the whole wafer into vibration, which causes several interfering effects. This phenomenon limits the applicability of vibrating diaphragms in resonant sensors.